1. Field of the Invention
The present invention relates to a magnetic recording medium that is used in an information recording apparatus of information processing equipment such as a computer or a recording apparatus of a consumer product. More particularly, the present invention relates to a magnetic recording medium used in a hard drive.
2. Description of the Related Art
With the recent increase in the amount of information handled by computers and other information processing equipment as well as downsizing of such information processing equipment, the storage capacities of the information recording apparatuses have increased, and the storage capacities required in magnetic recording media used in the information recording apparatuses keep steadily increasing.
FIG. 2 shows a cross-sectional schematic diagram of a magnetic recording medium. This magnetic recording medium 1 is generally configured by a substrate 2, a magnetic layer 3 on which information is recorded by a magnetic head, a protective layer 4 for protecting the magnetic layer from corrosion, wear, shock, and the like, and a lubricating layer 5 covering the surface of the protective layer 4. For the purpose of increasing the storage capacity of this magnetic recording medium and improving its recording performance, the distance between a read/write element of the magnetic head and the magnetic layer 3 of the magnetic recording medium, which is, in other words, a magnetic spacing, needs to be reduced to the utmost. The magnetic spacing is formed by the thickness of a protective layer of the magnetic head, the flying height of the magnetic head, and the thicknesses of the protective layer 4 and the lubricating layer 5 of the magnetic recording medium. A challenge of development of the magnetic recording medium is to reduce the thickness of the protective layer 4. Amorphous carbon called diamond-like carbon (DLC) has been generally used as the protective layer 4 of the magnetic recording medium.
A current magnetic recording medium has a recording density of approximately 500 gigabits/square inch, and the thickness of the protective layer 4 of the magnetic recording medium is more than 2.5 nm but not more than 3.5 nm. In order to increase the recording density to 750 gigabits/square inch or more in the future, it is considered that the magnetic spacing needs to be 8.5 nm or less. Considering the breakdown, the thickness of the protective film of the magnetic head needs to be 2 nm or less, the space between the outermost surface of the magnetic head and the outermost surface of the magnetic recording medium (flying height of the head) needs to be approximately 3 nm, the thickness of the lubricating layer of the magnetic recording medium needs to be 1 nm, and the thickness of the protective layer of the magnetic recording medium needs to be 2.5 nm or less. Increasing the recording density to 2000 gigabits/square inch in the future means that the thickness of the protective layer needs to be 1 nm.
On the other hand, the protective layer of the magnetic recording medium needs to be highly reliable. In other words, the protective layer needs to have sufficient corrosion resistance, sliding durability, and magnetic head flyability. The same level of reliability is required when reducing the thickness of the protective layer. However, if the protective layer of DLC made by a conventional method has a thickness of 2.5 nm or less, the properties described above are not reliable enough.
In order to address the issues described above, Japanese Patent Application Publication No. H9-138943 proposes the following. An underlayer made of Si, Ge, Sn, and the like is interposed between a magnetic film and a protective film, and the thus obtained layer is used as a buffer film to reduce residual strain of the protective film which is a carbon layer. This configuration can improve sliding durability of the magnetic medium and reduce the thickness of the protective layer, i.e., the integration of the buffer film and the protective film. Nevertheless, the thickness of the protective layer remains 5 nm, e.g., 7 in Example 2 of Japanese Patent Application Publication No. H9-138943, which still is too thick to achieve a target thickness of 2.5 nm or less. Moreover, it is admitted that theoretically speaking, the sum of the film thickness of the buffer film and the protective film at which the strain of the protective film is alleviated is over approximately 2.5 nm. Again, the configuration described in Japanese Patent Application Publication No. H9-138943 is not adequate to practically realize a film thickness equal to or less than 2.5 nm, see paragraph 0017 of Japanese Patent Application Publication No. H9-138943.
Japanese Patent Application Publication No(s). 2008-176915, 2008-192288, and 2008-234828 similarly propose double protective layers excellent in abrasion resistance and corrosion resistance. According to these patent documents, an aluminum oxynitride, a silicon oxynitride, and an oxynitride containing transition metals of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W are proposed as underlayers. Each of these underlayers can compensate for the stress of a protective film which is a carbon layer, so that the carbon layer can effectively adhere to a magnetic layer, achieving strong and stable bonding therebetween.
The issue in each of these patent documents, however, is that in the region where the thickness is reduced to 2.5 nm or less, the protective layer has micro-level coarseness. This causes a film thickness distribution in the lubricating layer and makes the magnetic head flyability unstable.
In addition, the DLC film needs to be denser in order to realize corrosion resistance and sliding durability in the thin protective layer. For example, according to Diamond and Related Materials, 3rd Ed., (1994), pgs. 361-368 by J. Robertson, a dense film with high sp3 bonding properties was developed by optimizing the energy of carbon ions generated by a plasma or by a high plasma density processing.
On the other hand, a dense DLC film of high corrosion resistance has high water repellency and provides less interactions, such as hydrogen bonding, chemical bonding, and polar interactions, with a terminal group of a lubricant. For this reason, the lubricant is known to have poor binding with the protective layer which is the DLC film. According to Japanese Patent Application Publication No. S61-222024, the protective film thereof with high corrosion resistance is defined by a water contact angle by taking advantage of the fact that the denser the film is, the higher the water repellency thereof. However, applying the lubricant to the protective layer when the bonding between the lubricant and the protective layer is poor, increases the ratio of non-bonded lubricant thickness to the total applied thickness, and, in a subsequent head flyability test, the non-bonded lubricant easily flows around the head in response to a wind pressure generated when the head flies, lowering the lubricity of a part where the head flies or preventing the head from flying stably due to transfer of the lubricant thereto.
As a countermeasure against such issues, Japanese Patent Application Publication No. 2001-266328, for example, describes that performing a nitrogen plasma treatment on the surface of the protective layer or an extremely shallow region from this surface, can reduce the contact angle of water to the protective layer, i.e., the water repellency of the protective layer. The nitrogen plasma treatment is a method for taking active nitrogen ions or nitrogen radicals into the surface of the protective layer by generating a plasma in a chamber into which nitrogen gas is introduced, in order to reduce the water repellency of the protective layer. The issues here are the film thickness of the protective layer and deterioration of the protective layer caused by the nitrogen plasma treatment. When the protective layer is as extremely thin as 2.5 nm or less, the nitrogen plasma treatment has an impact not only on the outermost surface of the protective layer but also on the entire protective layer. Therefore, the most part of the protective layer becomes damaged by the nitrogen plasma treatment, deteriorating the denseness and corrosion resistance of the protective layer. However, toning down the nitrogen plasma treatment weakens the adhesion of the lubricant, resulting in trade-off relationship where the protective layer is damaged or the head flies unstably. The prior art described above are verified only when the thickness of the protective layers exceeds 2.5 nm, which means that the entire protective layers cannot be damaged.
An object of the present invention, therefore, is to improve the recording performance by further reducing the thickness of the protective layer of the magnetic recording medium.